use std::borrow::Cow;

use smallvec::SmallVec;

use crate::{BStr, ByteSlice, CommitRef, parse, parse::ParseResult};

/// Parse the commit message after the header/message separator.
///
/// Typical input starts with the blank-line separator before the message, for
/// example `\nsubject\n\nbody\n`. The returned message excludes that first
/// separator newline and borrows all remaining bytes from `i`.
///
/// On success, `i` is advanced to the empty suffix as commits end with a message.
pub fn message<'a>(i: &mut &'a [u8]) -> ParseResult<&'a BStr> {
    if let Some(rest) = i.strip_prefix(parse::NL) {
        *i = &[];
        Ok(rest.as_bstr())
    } else {
        Err(crate::decode::Error)
    }
}

/// Parse a complete commit object body.
///
/// Typical input starts with `tree <hex>\n`, followed by zero or more
/// `parent <hex>\n` headers, then `author <signature>\n` and
/// `committer <signature>\n`. An optional `encoding <name>\n` header and any
/// number of extra single-line or multi-line headers may follow. The headers
/// are terminated by a blank line, after which all remaining bytes are the
/// commit message.
///
/// On success, the returned [`CommitRef`] borrows fields from `i` where
/// possible, and `i` is advanced to the empty suffix. Extra single-line header
/// values are borrowed, while multi-line header values are unfolded into owned
/// buffers.
///
/// This parser is not transactional as a whole: if a later required field or
/// the final message parse fails, `i` may already have been advanced past
/// earlier successfully parsed fields.
pub fn commit<'a>(i: &mut &'a [u8], object_hash: gix_hash::Kind) -> ParseResult<CommitRef<'a>> {
    let tree = parse::header_field(i, b"tree", |value| parse::hex_hash(value, object_hash))?;

    let mut parents = SmallVec::new();
    loop {
        let before = *i;
        match parse::header_field(i, b"parent", |value| parse::hex_hash(value, object_hash)) {
            Ok(parent) => parents.push(parent),
            Err(_) => {
                *i = before;
                break;
            }
        }
    }

    let author = parse::header_field(i, b"author", parse::signature_raw)?;
    let committer = parse::header_field(i, b"committer", parse::signature_raw)?;

    let before = *i;
    let encoding = match parse::header_field(i, b"encoding", Ok) {
        Ok(encoding) => Some(encoding.as_bstr()),
        Err(_) => {
            *i = before;
            None
        }
    };

    let mut extra_headers = Vec::new();
    loop {
        let before = *i;
        match parse::any_header_field_multi_line(i)
            .map(|(k, v)| (k.as_bstr(), Cow::Owned(v)))
            .or_else(|_| {
                *i = before;
                parse::any_header_field(i).map(|(k, v)| (k.as_bstr(), Cow::Borrowed(v.as_bstr())))
            }) {
            Ok(header) => extra_headers.push(header),
            Err(_) => {
                *i = before;
                break;
            }
        }
    }

    let message = message(i)?;
    if !i.is_empty() {
        return Err(crate::decode::Error);
    }

    Ok(CommitRef {
        tree,
        parents,
        author,
        committer,
        encoding,
        message,
        extra_headers,
    })
}